There are many characteristics to consider when optimizing for strength. Obviously, this is highly dependent upon the material properties of your filament. It is also highly dependent on the number of shells in the part, the infill shape, and the infill density.

We can optimize surface finish on any German RepRap 3D Printer with Simplify3D (our slicer software). Surface finish is a function of layer size and nozzle size. The smaller both are, the better it looks. The tradeoff is print time, the smaller the layer and nozzle size are, the longer it takes to print.